1. Technical Field of the Invention
The present invention relates to an aircraft or spacecraft based radar system with synthetic antenna aperture (SAR=Synthetic Aperture Radar) for imaging the earth""s surface in such a way that an ambiguity suppression is provided by means of a minimum antenna directionality with side lobe suppression resulting in a swath illumination on the ground.
2. Prior Art
By now, numerous SAR systems are used worldwide to image the earth""s surface. These systems may be implemented both mounted on an aircraft, as well as mounted on a spacecraft, e.g., on a satellite. In comparison to optical imaging, SAR systems are unable to use natural light sources, but must themselves illuminate the area to be imaged in the desired frequency range in a suitable manner.
In the known SAR systems this is achieved by using a single antenna for both the transmit and the receive operation. A SAR system of this type, which may also be referred to as a monostatic SAR system, requires a pulsed radar operation whereby the transmit pulses are limited in time, so that the time between every two successive pulses can be used for the receive operation. This radar mode has some principle-based shortcomings and limitations.
In the conventional monostatic SAR systems, as shown with the aid of an example in FIG. 1, the transmit pulses coming from a high power amplifier (HPA) 1 in the transmit branch are switched via an RF circulator 2 to an antenna 3, and emitted from there into space toward the ground. The transmit pulses are first processed in the transmit branch by a digital chirp generator 4 and routed via two quadrature channels I and Q with respective filters 5 and 6 and respective mixing steps 7 and 8 to an adder 9. Afterwards they are converted to the transmit frequency position by means of a mixing step 10, which is furthermore operated with the frequency of a local oscillator 11, after which they are filtered with the aid of an RF filter 12. Afterwards they are routed to the aforementioned high power amplifier 1. The radar echoes that arrive between successive transmit pulses are received by the same antenna 3 and switched via the HF circular 2 and a receiver protection circuit 13 in the receive branch to a low noise amplifier (LNA) 14.
After pre-filtering in an HF filter 15, mixing with the frequency of a local oscillator 16 in a mixing step 17, filtering into the baseband by means of a filter 18, sufficient amplification with the aid of an amplifier 19, and analog/digital conversion by means of an analog/digital converter 20, the standard processing methods for the SAR image generation are applied to the raw radar data that have been obtained in this manner.
The described SAR system of the monostatic type, which is known, e.g., from U.S. Pat. No. 4,866,446, and in comparable form for topographic mapping also from DE 37 12 065 C1, has a number of shortcomings, however.
The circulator that is necessary to use a single antenna results in losses and has only a limited decoupling between the transmit branch and the receive branch. In combination with the receiver protection circuit, which is therefore required, this leads to higher losses and a deterioration of the system noise factor. Also, the maximum peak transmitting power of monostatic systems is presently limited by the receiver protection circuit, which can be implemented only to a certain extent. Also of disadvantage are the complex power supply and the high EMC (electromagnetic compatibility) load on the total system, which results because high peak transmitting outputs become necessary in the pulse operation. The dimensioning of the transmit pulse length must take into consideration the requirements of the receive window. A reduction of the transmit pulse length, however, necessitates an increase in the transmitting power if a constant signal/noise ratio is to be maintained.
In the known monostatic SAR system, a limited flexibility of the total system furthermore results when special SAR operating modes are implemented, and when interferometric measurements are performed. Also, a monostatic SAR system is not capable of transmitting and receiving simultaneously. During the transmit operation a reception is not possible, and during reception of the radar echo no transmit pulse can be emitted. This limits the maximum time available for the scanning of a radar echo to a fraction of the transmit pulse interval. Since the transmit pulse interval, too, must not fall below a minimum value, which is dictated mainly by the resolution, a largest possible maximum image swath width exists, which is dependent mainly upon the required resolution and which cannot be exceeded.
For spacecraft based SAR systems, this maximum image swath width is approximately 8 to 20 km, depending on the incident angle, at a resolution of 1 m, or approximately 40 to 100 km at a resolution of 5 m.
The inability of the known monostatic SAR system to simultaneously transmit and receive furthermore means that those range domains cannot be imaged for which the time delay of the radar wave is an integral multiple of the transmit pulse interval. In order to still be able to image these range domains, the transmit pulse interval or the pulse repetition frequency (PRF) of the transmit pulses must be changed. However, during each such switching process, the end of the time delay of the radar wave must be waited for, which means system losses and which, ultimately, further limits the resolution that can be attained with the SAR system.
Since the incident angle and the angle of reflection are always the same in the known monostatic SAR system, and located on the same side of the perpendicular to the area to be imaged, the backscatter characteristics of the surface to be imaged can be measured only for identical incident and reflection angles, which means that a large part of the microwave characteristics of the surface to be imaged, therefore, cannot be registered by the known SAR system of the monostatic type.
It is true that a radar system with a synthetic aperture of the biostatic type is already known from JP 61-140 884 A, wherein a transmit antenna and a receive antenna are provided, which are physically separate and at least one of which, namely the transmit antenna, moves so that a relative movement results between the transmit antenna and the receive antenna. The transmit antenna, which is arranged on a moving platform, is located above the earth""s surface; however, the receive antenna with its related reception and evaluation system is not. Instead, the receive antenna is located on the ground or on a ship.
The receive antenna which is mounted stationary on the ground or on a ship, receives the portions of the signals that are reflected from a specific target object, which come from the transmit antenna that moves above the earth""s surface. The specific target object is also located above the earth=s surface, since a receive antenna mounted on the ground would not be able to receive meaningful reflection signals from a target object that is also located on the ground, for reasons of the usually present unevenness of the ground and the curvature of the earth""s surface alone. This SAR radar system, because of its design with a receive antenna mounted stationary on the ground or on a ship, is therefore not suitable at all for imaging ground structures, and its bistatic characteristics also cannot be transferred to a monostatic SAR radar system intended to image the earth=s surface.
It is the aim of the invention to create a SAR system for imaging the earth""s surface that permits, without significant system losses and with a high attainable resolution, a greater flexibility both in the image arrangement, as well as in the suppression of the described ambiguities, and permits a noticeable increase in the swath width. Furthermore, the SAR system to be created should be easy to implement and also result in some cost reductions.
In accordance with the invention, which relates to a radar system of the above type, this aim is met in such a way that the antenna is divided into a transmit antenna and a receive antenna according to the bistatic radar, which is known per se, and that these antennas are provided in physically separate locations, that the transmit antenna and the receive antenna are located above the earth""s surface to be imaged, on different platforms of which at least one is moving so that a relative movement results between the transmit antenna and the receive antenna, and that either the transmit antenna, the receive antenna, or both antennas are designed for ambiguity suppression.
With the bistatic SAR system according to the invention it is possible to transmit and receive at the same time. It is even possible to transmit and receive continuously, in such a way that the transmit signal is periodically repeated in certain time intervals (transmit pulse interval). The transmit signal may advantageously be a frequency modulated continuous (FMCW) signal. In that case the transmit signal may, for example, be frequency modulated in a saw-tooth pattern and traverses the entire system bandwidth before it jumps back to the starting frequency after the transmit pulse interval has ended.
However, other periodic modulation types that utilize the entire system bandwidth are possible as well. As with the monostatic SAR system, this transmit pulse interval results in local ambiguities. Ambiguity areas from which radar echoes reach the instrument offset by one or more transmit pulse intervals must be suppressed.
It is an objective of the antenna to suppress these ambiguities. In monostatic SAR systems there is only a single antenna. This antenna must reliably and with sufficient strength suppress the ambiguity areas from which the radar echoes reach the instrument at the same time and with the same Doppler shift as the radar echoes that come from the target area to be imaged. To this effect the antenna must maintain a certain minimum directionality, and the side lobes in the antenna directivity pattern must be sufficiently suppressed. The demand for a certain minimum directivity also means that only a certain swath width can be illuminated. The current satellite based SAR systems that work according to the monostatic principle are all designed according to this system limit.
In the bistatic SAR system according to the invention, a selection can first be made, as to which antenna (transmit antenna or receive antenna, or also both) is to suppress the ambiguities. The system can thus, in contrast to the monostatic principle, be designed such that the antenna lobe of the transmit antenna is so wide that the entire look area is illuminated from the smallest to the largest incident angles without having to pay attention to ambiguity zones. The suppression of ambiguity zones is then performed solely by the receive antenna or antennas. With a wide-swath system design like this, the demands on the transmit antenna, therefore, are only minor. One should merely make sure that no transmitting power is wasted to areas that cannot be imaged and that the swath to be imaged is illuminated fairly homogeneously. The transmit antenna in spacecraft based systems thus becomes comparatively small and economical.
The receive antenna, on the other hand, should generally be tapered to attain the side lobe attenuation of approximately 25 dB required for a good ambiguity suppression. In comparison to the transmit antenna, the receive antennas will thus have a large area, with high demands on the manufacturing accuracy. However, the receive antennas can be directed at random within the access swath illuminated by the transmit antenna. There are no dead zones as with the monostatic SAR system.
By putting together a plurality of swaths that are free of ambiguities in themselves the entire look area of the transmitter/receiver combination can, on principle, be imaged with the inventive bistatic SAR without limitation of the possible resolution and regardless of the applied pulse repetition frequency. For each of these swaths that are free of ambiguities in themselves, a separate antenna may be used, and also a separate platform, e.g., in the form of a satellite. However, the implementation is also possible with only one receive antenna, namely when a principle is used, which is related to the principle of the phased antenna array.
In the phased antenna array, the antenna is divided, in the elevation, into several lines. Each of these lines has its own adjustable time delay shifter, so that the signal time delay can be adjusted separately for each antenna line before the signals of the individual lines are added up to the antenna sum signal. With suitable adjustments of the time delay shifters, the direction of the main antenna lobe can be adjusted. If amplitude adjusters are added to the time delay shifters, the width of the main antenna lobe can be adjusted as well. In a physically implemented phased antenna array, a main antenna lobe can be precisely implemented that can be swiveled and shaped in a wide area. If it were desired to implement several electronic antennas that are oriented in different directions using, however, only one physical receive antenna, a separate time delay shifter and gain adjuster would be required for each of these electronic antennas. In that case the antenna would thus become extremely complex.